The proposed research is designed to investigate how the acoustic-phonetic information in the speech signal serves to activate a lexical representation for spoken word recognition. The research focuses on the dimension of acoustic-phonetic similarity and its consequences for lexical activation. The experiments use phoneme identification and reaction time measures. The goals of the research are to investigate the sufficiency of acoustic-phonetic similarity as a determining factor in spoken word recognition and the constraints on the use of acoustic-phonetic similarity imposed by various types of information (sequential structure, phoneme class, phonotactic and allophonic regularities, sentence context). While the majority of the proposed research will employ a normal college age subject population, the findings from the proposed research may provide a baseline against which to assess the consequences of the aging process for spoken language processing.